Rotating shafts in mechanical systems have myriad applications. For example, in the field of linear actuators, so called ball or lead screws may be used in conjunction with a ball or lead nut. In use, a prime mover (typically a motor) is connected to the ball or lead screw by means of a coupling. A corresponding ball or lead nut is mounted onto the screw but is prevented from rotating by virtue of an anti-rotation feature associated with the nut. Because the nut is captive and prevented from rotating, rotation of the screw causes linear motion of the nut along a longitudinal axis of the screw and, consequently, the linear actuator. Generally, the friction between the ball or lead screw and the corresponding nut is designed to be as low as possible in order to reduce the energy needed to induce the linear motion, which in turn permits the use of less powerful (and costly) prime movers.
FIG. 1 illustrates a typical use of a linear actuator. In particular, the linear actuator system 100 comprises a ball or lead screw 102 operatively connected to a prime mover or motor 104 via, in this case, an intervening worm gear box 106. The combination of a the motor 104 and/or gear box 106 may be considered a rotational drive source for the ball or lead screw 102. A ball or lead nut 108 may be mounted on the ball or lead screw 102 and connected to a load platen 112. As further shown, the vertically-oriented ball or lead screw 102 may be supported by a bearing 110 (illustrated in FIG. 1 through a partial cut-away view of the load platen 112) such that substantially all axial forces applied to the ball or lead screw 102 (through presence of a load) are borne by the bearing 110, as known in the art. The load platen 112, in turn, is maintained in alignment with the ball or lead screw 102, and may travel vertically under the guidance of, guide rails 114 that are typically mounted on a suitable wall or other support structure (not shown). Rotation of the ball or lead screw 102 causes linear translation of the ball or lead nut 108 that, in turn, induces vertical movement of the load platen 112. Any suitably sized load (not shown) to be moved vertically may be operatively coupled to the load platen 112.
Often, the low-friction relationship between the ball or lead screw 102 and the nut 108 may give rise to so-called “back driving” where a force applied by the load to the screw 102 via the nut 108 causes unwanted rotation of the screw (in a direction opposite the rotation provided by the rotational drive source) and linear displacement of the nut 108 and, consequently, the load. However, in applications such as that illustrated in FIG. 1, the low efficiency provided by the gear box 106 prevents back driving of the screw 102, i.e., the gear box 106 effectively serves as a brake against rotation of the screw 102 when the screw 102 is not otherwise being driven by the motor 104.
FIG. 2 illustrates a typical prior art ball or lead screw 102 of the type used in the system 100 of FIG. 1. More particularly, the ball or lead screw 102 comprises an input journal 202 through which a rotational drive source may be operatively connected to the ball or lead screw 102. The input journal 202 may comprise a keyway 204 configured to receive a key (not shown) used to secure the ball or lead screw 102 to the rotational drive source. The input journal 202 transitions to a threaded region having a thread form 206 formed therein, beginning with a terminal portion 207 of the thread form 206. As known, the ball or lead nut 108 complementarily mates with the thread form 206. The input journal 202 has a reduced diameter relative to the threaded region, and the transitional region 208 between the keyway 204 of the input journal 202 and the threaded region is a known stress concentration area that is susceptible to failure, e.g., torsional fracture or shear. In the event of a failure within such a region of weakness, the support provided to the ball or lead screw 102 by the bearing 110, along with the maintenance of the load platen 112 by the guide rails 114, will typically prevent the load from immediately dropping. However, because the screw is no longer coupled to the gear box 106, such a failure may permit back driving of the threaded region of the screw 102 and movement of the load (i.e., dropping under the force of gravity), potentially with catastrophic results.
Thus, it would be advantageous to provide solutions to the above-described shortcomings.